Combined Effect of Vitamin D Status and Physical Activity on Urinary Flow Rate: A Cross‐Sectional Study

OBJECTIVES: To explore the independent and combined effects of vitamin D status and physical activity on urinary flow rate (UFR), a key indicator of voiding function. METHODS: We analyzed data from 17 670 participants in the National Health and Nutrition Examination Survey (2009-2016). Physical activity status was assessed via self-reported weekly moderate-to-vigorous recreational activity. Vitamin D status was evaluated by measuring total serum 25-hydroxyvitamin D [25(OH)D] levels. Weighted linear regression models and restricted cubic spline (RCS) regression models were used to examine associations between serum 25(OH)D, physical activity, and UFR. An XGBoost model was applied to assess variable importance. Stratified analyses and interaction tests were performed to assess the combined effects of serum 25(OH)D and physical activity on UFR. RESULTS: Higher serum 25(OH)D levels were significantly associated with increased UFR. RCS analysis indicated a nonlinear relationship between serum 25(OH)D and UFR (p for nonlinear = 0.0426). Active physical activity was positively associated with UFR (β = 0.085, 95% CI: 0.053-0.116, p < 0.001). XGBoost model confirmed serum 25(OH)D and physical activity as key predictors of UFR. A significant interaction was observed between serum 25(OH)D levels and physical activity status in relation to UFR. Stratified analyses revealed a stronger positive linear association between serum 25(OH)D and UFR in participants with active or vigorous physical activity, while an inverted L-shaped relationship was found in the inactive group. CONCLUSIONS: Higher serum 25(OH)D levels were associated with increased UFR, particularly when combined with active or vigorous physical activity. These findings provide a scientific foundation for developing integrated interventions that combine nutritional supplementation and physical activity to manage lower urinary tract symptoms.